1. Field of the Invention
Embodiments of the present invention finds particularly advantageous but not exclusive application in the field of medical imaging and medical diagnostic apparatuses. These diagnostic apparatuses are X-ray image acquisition apparatuses. The Embodiments of the invention can nevertheless be applied to any other field in which a correction is undertaken of latent charges in a flat-panel detector.
2. Prior Art
Today, X-ray apparatuses can be used to obtain images, and even image sequences, of an organ situated within a living being, especially a human. An X-ray apparatus comprises an X-ray generator tube and an X-ray detector. The X-ray detector is a large flat-panel detector. This detector has a detector plate comprising photodiodes used to detect electromagnetic radiation, namely the X-rays. This detector plate is covered with a hood or cover. This cover is that element of the detector liable to come into contact with the patient during a radiology examination.
In a radiology examination, the patient is placed between the generator tube and the cover of the detector. An X-ray beam emitted by the generator tube is directed toward the patient.
During the radiology exposure, X-ray photons are absorbed by the patient's body in varying degrees. The rest of the X-ray beam, going through the patient's body, is detected by the detector plate of the detector. The photodiodes of the detector plate create a charge whenever its different points are stimulated by the residual rays. The detector has charge collector electrodes used to collect the charges which are then temporarily stored in capacitors. Under the effect of a control signal, the charges stored are transmitted to different TFT matrices. The signals thus obtained are amplified and then transmitted to the signal processing and image reconstruction computational units of the X-ray apparatus.
The images thus obtained are interpreted by a specialist practitioner in order to perform diagnosis or to assist in surgical operations and/or take action to treat the pathologies detected.
However, this type of X-ray apparatus has drawbacks. Indeed, a second source of residual signals is observed in the course of the acquisitions, thus causing deterioration in the quality of the images obtained with this type of apparatus. This second source of signals results from the history of the illumination of the photodiodes. This is a phenomenon called lag or persistence.
This lag is manifested by the fact that the intensity of the signal associated with a pixel depends on one or more previous exposures to X-rays. This phenomenon of lag is due to the fact that present-day X-ray apparatuses use amorphous silicon detectors. Owing to the nature of the amorphous silicon in the panel detector, the photodiodes contain traps that get filled by excitation from the X-rays and then get emptied by a process of decay with a relatively big time constant. As a result, a decaying image is retained by the detector. The importance of the lag image in the X-ray detectors decreases with time as the traps get emptied by thermal effect so much so that the persistence signal will decrease slowly until it is no longer visible. This decay may last several minutes.
The use of a big flat-panel X-ray detector thus causes problems of lag charges which disturb the images viewed by the practitioner. These latent charges which are visible in the last image acquired can smear said image thus causing errors of diagnosis.
In the case of vascular applications, several radiology images are acquired per second. Owing to the release of the charges trapped in the photodiodes, the previously acquired images disturb the next images that are to be acquired. And when the X-ray apparatus is mobile, the superimposition of the signal given by the latent charges and the correct signal may completely modify the result of the image viewed.
There are now several classic solutions used to resolve the drawbacks due to the trapping of charges in the photodiodes of the detector. One of the main, classic solutions consists in defining a constant error correction module to be applied to the radiology image. However, this approach has drawbacks for it produces optimal results only when the acquisition mode is the same and when the integration time of the detector is constant.
The acquisition mode may be a low-dose mode in which the intensity of the rays emitted by the tube is low and/or a high-dose mode in which the intensity of the X-rays emitted is high. The integration time is the time in which the charges accumulate in the photodiodes.
During a passage made from one acquisition mode to another, the predefined error correction model is no longer suited to resolving a problem of latent charges. This is because the fact that the quantity of latent charge stored is completely different from one mode to another.
Furthermore, in the prior art, the acquisition modes work according to a predefined succession of detector discharge times and integration times. It is not possible to vary the integration time of the detector. However since, in practice, the image acquisition operations have to be done at fixed positions and since the practitioner directs the positioning of the detector, the integration phases of the detector cannot be fixed. They are variable. The integration time lasts for a period of time whose duration varies when compared with the fixed time as defined in the prior art. The predefined error correction code is therefore not better suited to correcting the errors induced by the latent charges with variable integration times.